Food Distribution and a Variable Mating System in the Dunnock, Prunella modularis Author(s): N. B. Davies and A. Lundberg Source: Journal of Animal Ecology, Vol. 53, No. 3 (Oct., 1984), pp. 895-912 Published by: British Ecological Society Stable URL: http://www.jstor.org/stable/4666 Accessed: 30/10/2010 23:24 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=briteco. 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LUNDBERG* Department of Zoology, Universityof Cambridge,Downing Street, CambridgeCB2 3EJ SUMMARY (1) The mating combinations were very varied and included monogamy (pairs), polyandry (two or three males with one female), polygyny (one male with two females) and polygynandry (two or three males shared two, three or four females). (2) Female ranges were always exclusive. Where two or three males shared one or more females, the ranges of the males overlapped and they cooperated to defend the territory. There was a dominance hierarchy among the males; alpha males were often old birds and beta males often first year birds. There were no known cases of close relatives being involved within any of the mating combinations. (3) It is suggested that the differentmating combinations form a continuum in a male's ability to monopolize access to females, varying, in increasing order of mating success, from unpaired, to shared access to one female (polyandry), sole access to one female (monogamy), shared access to more than one female (polygynandry) and finally sole access to more than one female (polygyny). (4) The ability of a male to control access to females depended on female range size, which was influenced by food distribution.Where food patches were dense, female ranges were small and they were then easily monopolized giving rise to mating combinations that reflected high male mating success (polygyny and polygynandry). Where food patches were sparse, female ranges were large and they were difficult for one male to monopolize, thus giving rise to mating combinationswith lower male mating success (polyandry). (5) When extra food was provided on some territories, female ranges became smaller and the mating system shifted towards greater male mating success (from polyandry towards monogamy and polygynandry). INTRODUCTION In many species a female's reproductive success is limited by her access to resources whereas a male's reproductive success is limited by his access to females (Darwin 1871; Trivers 1972). Mating systems, therefore, should often be related to the ability of males to control access to females. The degree of control will depend on two main factors (Emlen & Oring 1977). Firstly ecological conditions such as the dispersion of food, nest sites and predators will influence female distribution which, in turn, will determine their economic defendabilityby males. Secondly, the ability of males to monopolize females will depend on the amount of competition for mates at any one time, which will be reflected by the operational sex ratio (the local ratio of receptive females to sexually active males). Three main methods have been used to test these ideas. The comparative approach across different species has shown that, as predicted, differences in mating systems are * Presentaddress: Department of Zoology,University of Uppsala,Box 561, S-751 22 Uppsala,Sweden. 895 896 Dunnock mating systems linked to resource dispersion and the temporal availability of mates (e.g. primates, Clutton-Brock & Harvey 1977; Wrangham 1980; ungulates, Jarman 1974; birds, Crook 1964; Verner & Willson 1966; frogs, Wells 1977; Arak 1983). Observations of variations within a species have likewise shown a correlationbetween a male's mating success and his ability to control access to females or resources that the females require (e.g. nest sites, Verner 1964; Pleszczynska 1978; food, Wittenberger1980). The thirdmethod, experimental manipulationof the factors supposed to influencethe mating system, has been rarely used. Two experiments have tested directly the idea that resource distribution influences polygyny. Pleszczynska & Hansell (1980) increased the number of females in male lark bunting (Calamospiza melanocorys)territories by providing extra nest sites while Ewald & Rohwer (1982) did likewise for male red-wingedblackbirds (Agelaius phoeniceus) by the provision of extra food. One experiment has investigated the importance of mate competition. By removing males from a population, Smith, Yom-Tov & Moses (1982) showed that the operational sex ratio can influence the mating system of song sparrows (Melospiza melodia), a typically monogamous species; the degree of polygyny increased because the widowed females formed associations with already mated males. Here we describe observations and an experiment to investigate the ecological factors that influence the mating system of the dunnock (or hedge sparrow, Prunella modularis L.). The species is of unusual interest because it has a very variable mating system with monogamy, polygyny, polygynandry and polyandry often all occurring together within the same population (Birkhead 1981; Karanja 1982; Snow & Snow 1982; Davies 1983). This variabilityprovides a good opportunityto explore the eco-correlates of mating systems and to test by experiment how changes in ecological conditions can influence females' disper- sion and their defendabilityby males. STUDY AREA AND METHODS The study area is the CambridgeUniversity Botanic Garden, an area of 16 hectares which includes a diversity of habitats. There are areas of woodland with dense undergrowth,open woodland with little undergrowth,hedgerows (mainly hawthorn Crataegus and evergreens Taxus, Thuja), flowers beds, shrubs, long grass, dense patches of giant hogweed (Heracleum mantegazzianum) and areas of open lawn. The result is a patchwork of different vegetation types and densities which provides an ideal natural experiment for observing on a small scale how habitat influencesbird distribution. The dunnock population has been studied since October 1980. In the first breeding season, 1981, an area of 7.7 ha was studied but in subsequent years the study area has included the whole garden. Almost all of the birds were individuallyrecognizable by colour rings (forty-five out of the forty-nine breeding adults in 1981, seventy-two out of seventy-seven in 1982 and eighty-one out of eighty-six in 1983). Most of the nests were built in hedges and evergreen bushes and were easy to find. All the nestlings were colour ringed (forty-five in 1981, 116 in 1982 and 159 in 1983). Some remained to breed in the garden but many disappeared soon after independence, presumably dispersing to other areas. In late summer and autumn there was an influx of young birds born outside the study area. Some remainedto breed the next summerbut many left after a brief visit. These immigrants were easily recognized by their pale legs and dark brown eyes (adults have red-brown eyes) and were also colour ringed (forty-two in 1981, thirty-six in 1982). Wing lengths were measured from the carpeljoint to the tip of the longest primary,with the wing closed and pressed flat against a rule held along the long axis of the bird's body. Tarsus lengths were measured, with vernier callipers, from the notch in the angle of the intertarsal N. B. DAVIESAND A. LUNDBERG 897 joint to the tip of the bended foot. This measure is slightly larger than the true tarsus (Svensson 1970) but it is easier to take and more repeatable. Females were easily distinguished from males both by plumage (females were less grey on the head and underparts) and behaviour (females very rarely sang, males did not incubate and the differencein behaviourduring copulation was very distinctive;Davies 1983). Transects were made daily in the breedingseason and two-five times a week in the winter and the positions of individualswere marked on a map. Range areas were calculated by the maximum polygon method (Odum & Kuenzler 1955) and male song territories were calculated as the area within the maximum polygon drawn around the male's regular singing perches. Occasionally birds wandered a long way outside their territories (e.g. to drink at a pond) and these map registrations were ignored when the maximum polygons were drawn. Most adults were sedentary and the boundaries of their regular home range were easy to draw. Continuous watches were made of individualsfor periods of 10-60 min to record time budgets; activities were categorized into time spent feeding, perching (including song), preeningand interactionswith others. In both 1982 and 1983 we put out food on ten randomly chosen territoriesto investigate the influence of extra food on time budgets and ranges. In each territory, food was placed in and around a wire mesh cylinder about 35 cm in diameter and 30 cm in height.